The present invention generally relates to a copying apparatus or the like and more particularly, to a heat fixing device for use in the copying apparatus or the like, in which a drop of temperature of the heating roller can be prevented.
Conventionally, in a copying apparatus including a fixing device composed of a pressing roller and a heating roller provided with a heater, there is a drawback that since transfer papers are cold and a large amount of heat of the heating roller is absorbed by the pressing roller in the case where the copying apparatus is operated under low ambient temperatures not exceeding 10.degree. C., automatic feed copying of an original document to produce a plurality of, for example, 10 transfer papers or more lowers the temperature of the heating roller dramatically as shown in FIG. 1. FIG. 1 illustrates temperature characteristics of the heating roller, i.e. changes of the temperature of the heating roller with time. The heating roller is maintained at a high fixing temperature of 180.degree. C. by a heater. However, the temperature of the heating roller gradually drops upon automatic feed copying of the original document to produce a plurality of transfer papers and finally, during a time period A, drops below a temperature (indicated by a one-dot chain line) leading to improper fixing of an image on the transfer paper.
In order to obviate such a drawback of the known heat fixing device, there has been proposed a heat fixing device in which a heater of a heating roller has an increased rated heating electric power so as to compensate for drop of temperature of the heating roller in the case of operation of the copying apparatus under low ambient temperatures of the copying apparatus. To this end, the heater of the heating roller has a rated electric power of, for example, 1.2 kW so as to heat the heating roller to the fixing temperature of 180.degree. C. in about 30 sec. when the ambient temperature of the copying apparatus is 20.degree. C. However, when the heater having such a high rated electric power as described above is used in the case where the copying apparatus is operated by a general commercial electric source, such a problem arises that since a light source lamp of the copying apparatus is turned on during a copying operation of the copying apparatus, electric power consumed by the copying apparatus becomes larger than the electric power supplied from the commercial power source so as to exceed a rated capacity of the receptacle, etc. Therefore, in this prior art heat fixing device provided with the heater having a large rated electric power, since actuation of the heater is required to be temporarily stopped so as to prevent the power consumption of the copying apparatus from exceeding the electric power supplied from the commercial power source during the copying operation of the copying apparatus, it is impossible to maintain the heating roller at the fixing temperature at all times.
In order to maintain the heating roller at the fixing temperature, there has been proposed, in addition to that referred to above, another heat fixing device shown in FIG. 2. This known heat fixing device includes a heating roller 1 provided at its central portion with a heater 3, and a pressing roller 2. A thickness t1 of an aluminium layer 4 of the heating roller 1 is set as large as 3 to 5 mm so as to improve the heat insulating property of the heating roller 1. When the thickness of the aluminium layer 4 is increased as described above, the amount of heat dissipated from the heating roller 1 is reduced. However, in the field of copying apparatus or the like, a keen demand for the reduction of the waiting time period of an operator lead to a recent trend for a decrease of the warm-up time period of the copying apparatus or the like. However, since this known arrangement of FIG. 2 is designed to secure the fixing temperature by increasing the thickness of the aluminum layer 4, a long time period is required for raising the temperature of the heating roller 1 to the fixing temperature of 180.degree. C., thereby resulting in an increase of the warm-up time period of the copying apparatus.
Referring to FIG. 3, there is shown a further prior art heat fixing device. The prior art heat fixing device includes an upper roller 1' provided with a heater 3' and a lower roller 2' provided with a heater 5. In this prior art heat fixing device, although a thickness t2 of an aluminium layer 4' of the upper roller 1' is not required to be increased to such an extent as in the known arrangement of FIG. 2, both the upper and lower rollers 1' and 2' are, respectively, provided with heaters 3' and 5, thus resulting in a rise of its production cost. This prior art heating fixing device further has such a disadvantage as large power consumption of the heaters 3' and 5.